An optical clock based on a highly charged ion

authored by
Lukas Josef Spieß
supervised by
Piet Oliver Schmidt

Optical clocks have demonstrated the lowest uncertainties among all experimental devices with applications in time keeping as well as for tests of fundamental physics. Their exceptional accuracy is realised by referencing their frequency to an electronic transition in either neutral or singly-charged atoms. The achieved uncertainties are often limited by external perturbations shifting the measured transition frequency. Highly charged ions (HCI) are intrinsically less sensitive to external-field perturbations, making them interesting candidates for such an application. The construction of a HCI-based optical clock was for along time prohibited by the megakelvin temperatures at which HCI are produced. Only in recent years, isolation and sympathetic cooling of individual HCI in a Paul trap has been achieved, which enables the application of quantum logic spectroscopy (QLS), resolving a narrow transition with Hz-level accuracy. In this thesis, the first optical clock based on a HCI is presented. For this, QLS is used to coherently excite the 2P1/2 - 2P3/2 transition at 441 nm in Ar13+. The large charge-to-mass ratio mismatch between the employed ions (40Ar13+, 9Be+), leads to challenges for cooling of some of the motional modes. This is overcome by employing a novel algorithmic cooling protocol, leading to the lowest temperature reported for a HCI. A detailed evaluation of the experimental setup yields a systematic uncertainty of 2 × 10^−17 comparable to many optical clocks in operation. A path to an uncertainty below 10^−18 is discussed and can be achieved with technical improvements. An optical clock comparison was performed and the derived absolute frequency of the clock transition improves its uncertainty by eight orders of magnitude. A nine orders of magnitude improvement of the isotope shift (40Ar13+ - 36Ar13+) resolves the quantum electrodynamical nuclear recoil, an effect not previously observed in a many-electron system. The Ar13+ clock is limited by its statistical uncertainty, which can be overcome by employing species offering transitions with narrower linewidth. However, their experimental determination remains challenging in the absence of data from fluorescence measurements. To aid with this, an optical dipole force technique is demonstrated, which is initial-state preserving and achieves broader linewidths than conventional Rabi interrogation. This will allow a plethora of clock transitions in HCI to be experimentally identified before employing them in optical clocks.

QUEST-Leibniz Research School
Doctoral thesis
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